1. Field of the Invention
This invention relates to the field of coatings, and is particularly concerned with a coating for sealing carbon-carbon composites against hydrogen.
2. Description of the Prior Art
Carbon-carbon composites have attained importance for use as aircraft structural materials, particularly on engines, e.g. exit nozzles, and airframes. Such composites are based on carbon fibers woven and formed into a structure, wherein carbon is both the fiber and the matrix forming the composite. The matrix can be a combination of graphitic carbon and amorphous carbon in certain percentages. Such composites can be produced in known manner by impregnating a carbon fiber structure with a resin, and heating the fiber impregnated resin at temperature sufficient to decompose the resin matrix and to volatilize all of the components of the resin except for the carbon.
In certain areas of aerospace use such as internal heat exchangers wherein the fuel is cryogenic hydrogen, such composites are placed into contact with the hydrogen. Carbon-carbon composites have potential for high temperature service. However, above 350.degree. C. hydrogen can react with carbon to form methane, which degrades its properties. To the applicants' knowledge, this problem has not been satisfactorily addressed to date in the prior art.